Solar ponds are a radically different inexpensive approach to building a solar collector. They are usually constructed using clear, stratified salt water usually two to three meters deep and varying from nearly saturated brine at the bottom to nearly fresh water at the top. Because of this salt gradient, lower layers remain too dense to rise through the lighter overlying layers, even when heated. That means convection is suppressed. Much of the incident sunshine penetrates to the pond between where it warms the adjacent brine layers to 70.degree. to 110.degree. C., that is, until heat losses neutralize further temperature increase. The pond's upper layers thus act as insulation for the lower layer and underlying ground.
The predominant heat loss is conduction upward through the pond driven by the established thermocline. Heat is also conducted into the ground below a new pond until this ground becomes well heated and this loss ceases. There can be losses to ground water currents flowing beneath a pond and sometimes these may need to be controlled. There are losses by conduction around the pond edges. Usually very expansive ponds are designed. Then edge losses (proportional to perimeter) become minor compared with conduction losses upward through the pond layers (proportional to area).
Solar ponds actually develop three zones. The surface zone (0.2 meters deep) is stirred by the wind and has an essentially uniform salt concentration and temperature. Below this is a well stratified convection suppressing zone in which density, temperature and salt concentration increase with depth. This is sometimes called the thermohalocline. Daily fluctuating pond floor temperatures stir the bottom zone so it also has a uniform temperature and salinity.
To extract heat from ponds, the bottom zone is decanted slowly enough so that it does not stir the overlying nonconvecting zone. The heat from this brine could then be used for space heating, industrial process heating, salt production, desalinization, in absorption airconditioning or Rankine cycle power generation.
Small ponds will usually be constructed by bulldozing an embankment around the pond floor area and laying a rubber or plasticized liner across the floor and embankments. The stratified layers can then be developed by first filling the pond halfway with brine, then sliding a fresh water layer on top of this. Salt diffusion gradually produces the even stratification.
Often the nearly saturated, very hot, bottom zone of the pond is intentionally made thicker to augment heat storage. The ground underlying a pond contributes additional thermal storage. As a rule of thumb, the ground storage effectively penetrates to one meter, a depth determined by heat conduction rates and the likely lenght of storage cycles. This intrinsic storage makes solar ponds reliable and responsive to changing consumer demand. Heat can be withdrawn to match almost any demand, day, night, or in bad weather. In contrast, lack of thermal storage and reliability cripples many other solar strategies.
This method of increasing thermal storage in the bottom of solar ponds is expensive, and therefore not included to the extent which could be used. At many potential pond sites brine is already expected to be the biggest contributor to costs. Brine cost is especially high when solid salt must be brought to a site by truck. This will commonly be necessary for most ponds designed to provide space, water, or process heating. Since the bottom zone of ponds contain a nearly saturated brine, we simply compound the salt requirements and cost problem for these pond types if we thicken this zone to augment thermal storage.
Moreover, greater thermal storage could often be used than economics will allow. For example, greater storage would further smooth out variations in diurnal demand, in the supply of sunshine, or caused by long stretches of bad weather. In addition, much vaster storage could neutralize the summer to winter variation in solar energy incident on ponds. This annual variation increases acutely with distance from the equator. Also, some types of demand will be seasonal, for example, space heating. Vast thermal storage could actually save summer solar energy for use in the next winter.